Many lasers are capable of operating at a number of wavelengths, either due to a broad band of emission such as in dye or in many solid-state lasers, or due to multiple discrete lines such as in molecular lasers like CO.sub.2. Tunable lasers have broad application in spectroscopy and in optical communications. In these applications rapid electronic control of wavelength is an advantage. High-speed tuning is particularly advantageous for remote sensing in that it provides a higher rate of data acquisition, and allows spectroscopic measurements to be achieved in short times compared to the time for platform motion or for atmospheric changes. Tuning rates of greater than 100 kHz are desirable.
Rapidly tuned lasers have most often used small galvanometer-driven mirrors or gratings to randomly access wavelengths at rates up to about 200 Hz or have used rapidly rotating mirrors or prisms to sequentially tune through wavelengths at speeds up to 40 kHz in bursts at a few hundred Hz. However, tuning speeds remain limited by inertia, mechanisms are bulky and difficult to align, and higher speeds and random access are difficult to achieve.
Acoustooptic (AO) devices have been used to replace these slow-moving parts with faster devices which are electronically controlled and have no moving parts. In Design and Fabrication of Acousto-Optic Devices, A. P. Goutzoulis and D. R. Pape, eds., Marcel Dekker, Inc. (New York, 1994) a useful review of a variety of AO devices, including modulators (AOMs), deflectors (AODs) and AO tunable filters (AOTFs) may be found.
An AOM is a device generally having a single acoustic transducer for generating a radio-frequency (RF) sound wave propagating approximately transverse to the optical beam path therethrough, and is usually used for modulating the intensity of light. This modulation occurs by varying the intensity of the RF signal to the transducer and thus the fraction of light diffracted out of the original (zero-order) direction into the first-order beam. For this application, usually the carrier frequency is fixed, and the directions of zero- and first-order beams relative to the acoustic-wave propagation are fixed. By contrast, the function of an AOD is to vary the deflection angle of the beam by varying the radio frequency. However, in order to maintain efficiency as the angle of the first-order output beam varies, it is customary to change the direction of propagation of the acoustic beam by using a phased-transducer array. An AOTF utilizes a birefringent crystal and an acoustooptic interaction between light of different polarizations. That is, input light of one polarization is converted to another polarization only for a narrow wavelength range determined by the acoustic frequency, with the filtered light selected by a polarizer or, in some geometrical situations, by a different propagation direction in the birefringent crystal. Acoustooptic tunable filters can be classified as collinear or noncollinear depending on whether the light and acoustic waves propagate in the same direction.
Acoustooptic tuning of a dye laser was first accomplished using an AOTF by Taylor et al., and reported in "Electronic Tuning Of A Dye Laser Using The Acousto-Optic Filter," Appl. Phys. Lett. 19, 269 (1971). In "Rapid Acousto-Optic Tuning Of A Dye Laser," by Lynn D. Hutcheson and R. S. Hughes, Appl. Opt. 13, 1395 (1974), a dye laser was tuned using an AO deflector to scan the laser beam across a grating. However, on each pass through the AO device, the diffracted beam is frequency shifted relative to the input beam by the acoustic frequency, with the direction of the shift depending on the relative orientation of the optical and acoustic beams. This shift accumulates as the light makes multiple passes of the cavity and prevents single-frequency operation (See, e.g., "Analysis Of A Dye Laser Tuned By Acousto-Optic Filter," by William Streifer and John R. Whinnery, Appl. Phys. Lett. 17, 335 (1970)). In the case of narrow gain lines, such as in low-pressure CO.sub.2, if the gain is low or many round trips inside a laser cavity are required, the frequency shift prevents laser output altogether.
A number of approaches have been proffered for eliminating this shift. In "Optical Oscillator Sweeper," U.S. Pat. No. 5,263,037, which issued to William R. Trutna and Paul Zorabedian on Nov. 16, 1993, two AOTF's were inserted into the laser cavity and operated so that their frequency shifts were of opposite sign and canceled each other. In "Electronically Tunable External Cavity Semiconductor Laser," by G. A. Coquin and K. W. Cheung, Electron. Lett. 24, 599 (1988), an intracavity AOTF and an AO modulator were employed, the AOTF providing the tunability and the modulator serving only to compensate for the frequency shift of the AOTF. In "Rapid Tuning Mechanism For CO.sub.2 Lasers," Proc. SPIE 894, 78 (1988), L. J. Denes et al. used a single collinear AOTF to tune a TEA CO.sub.2 laser. The acoustic wave was reflected at one end of the AOTF to create counterpropagating acoustic waves. Two passes of the laser light through the AOTF produced components at the unshifted frequency and at up- and down-shifted frequencies. While the one-half of the light that is unshifted in frequency allows operation on the relatively narrow CO.sub.2 lines, a 50% round-trip loss is experienced that may not be acceptable for lower-gain lasers.
Acoustooptic tunable filters as laser tuners have some limitations, particularly in the infrared where suitable crystals are not widely available. Acoustooptic deflectors and modulators have been used as alternatives. In "Electron Tuning Of LEC Lasers," by K. Doughty and K. Cameron, Proc. SPIE 1703, 136 (1992), a pair of AODs was used to cancel the frequency shifts, the AODs being oriented such that the deflection angles also canceled in whole or in part. In one configuration, two AODs constructed of different materials were used in cooperation with a grating so that the frequency shifts canceled exactly, but a residual deflection remained. However, only limited tuning range was reported. A proposed configuration would use identical deflectors, with exact cancellation of frequency shift and deflection angle, to produce a frequency-dependent displacement, which would be converted by a lens to an angular deflection at a grating. In "Rapid Wavelength Switching Of IR Lasers With Bragg Cells," U.S. Pat. No. 4,707,835, which issued to Hans W. Mocker on Nov. 17, 1987, a ring laser having an additional coupled cavity containing one double-passed AO modulator was proposed. This coupled cavity adds significant complexity. Moreover, while Mocker claims that limited AOM efficiency makes the coupled cavity necessary, the disclosed configuration requires high AOM efficiency to prevent lasing at unselected higher-gain wavelengths. A different configuration proposed by Mocker uses two separate AO devices deployed in series to leave the signal frequency unshifted, but would be expected to provide poor wavelength resolution due to the cancellation of the deflection angles. That is, the light is simply displaced by the two devices, with individual wavelengths simply retracing their paths after reflection from a cavity mirror. An iris inserted into the laser cavity next to the totally reflecting mirror may provide limited resolution of the laser wavelengths.
Accordingly, it is an object of the present invention to provide a laser which is capable of rapid, high-resolution output wavelength tuning and cavity parameter control with the use of highly efficient acoustooptic elements and simple optics.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.